The inability of new V.90 modems such as the Acceleport RAS to connect with older legacy modems is typically caused by a failure to negotiate LAPM error correction. Most of the modems where this type of problem occurs are modems which were either manufactured before or very soon after LAPM (V.42) error correction was available. It is this reason that a modern V.90 modem and an older modem may have trouble connecting if LAPM is enabled on the V.90 modem. It should also be noted that upgrading the firmware on the older modem (if upgrade is available) will typically resolve such a problem. Since firmware upgrades for legacy modems aren't always available, we''ve added an AT command to our command set which will hopefully provide a workaround for our customers when added to a dialing or initialization string.
- AT~V7=x, where x=5-30 (default 30) seconds.
~V7=x represents a configurable LAPM negotiation phase timeout value. At the default of x=30, if some modem and the Acceleport RAS don't negotiate LAPM error correction within 30 seconds, we will stop trying and instead fallback to either MNP error correction or normal mode (no error correction) depending how /N is set. The reason that the default value of 30 may cause problems when connecting with a legacy modem is that S-register S7 (carrier detect abort timer) is typically set for a value of 30-45 seconds on most modems. The modems are aborting their attempted connection because they didn't agree on an error correction protocol within their alloted S7 time period to establish carrier! To resolve this type of issue, experiment with a lower value of ~V7 such as 15 or 20, so the modems will move on to negotiate some other protocol before the S7 time period expires. If setting the LAPM negotiation timeout as mentioned above doesn't resolve the issue outright, here's a dialing string with some other commands to help "dumb down" a modem and enable it to connect to older modems:
- AT - begins all modem command strings
- &F - Factory default, aka known starting point to configure from
- \N1 - Disables error correction (equivalent to setting &M0&Q0)
- S95=45 - Extended result codes (not necessary, but gives useful output)
- %C0 - Turns off Data Compression
- %E0 - Disable line quality monitor and auto-retrain
- +MS=9,1,300,9600 - V.32 modulation, 300 baud min, 9600 baud max speed
- DT - Tone Dial (phone number)
For more information:
All AT commands and their options are described in detail in the AT Command Reference manual:
Jun 11, 2019